1. Field of the Invention
The present invention relates to a technique for electrochemically hydrogenating an aromatic compound.
2. Description of the Related Art
It is known that a cyclic organic compound such as cyclohexane or decalin is efficiently obtained by hydrogenating a benzene ring of a corresponding aromatic hydrocarbon compound (benzene or naphthalene) using a hydrogen gas. This reaction requires reaction conditions of high temperature and high pressure, and thus is unsuitable for small to medium scale manufacturing a cyclic organic compound. On the other hand, in an electrochemical reaction using an electrolysis cell, it is not necessary to treat gaseous hydrogen since water can be used as a source of hydrogen, and also the reaction is known to proceed under relatively mild reaction conditions (at from room temperature to about 200° C. and under normal pressure).    [Patent document No. 1] JP 2003-045449    [Patent document No. 2] JP 2005-126288    [Patent document No. 3] JP 2005-239479    [non-patent document No. 1] Masaru Ichikawa, J. Jpn. Inst. Energy, vol. 85, 517 (2006)
As an example of electrochemically hydrogenating a benzene ring of an aromatic hydrocarbon compound such as toluene, a method has been reported in which toluene that is vaporized into a gaseous state is sent to the reduction electrode side to obtain methylcyclohexane, in which a benzene ring is hydrogenated, without going a state of a hydrogen gas, in a configuration similar to that of water electrolysis (see Masaru Ichikawa, J. Jpn. Inst. Energy, vol. 85, 517 (2006)). However, the amount of substance that can be transformed per electrode area or time (current density) is not large, and it has been difficult to industrially hydrogenate a benzene ring of an aromatic hydrocarbon compound.
As a method for improving the above-problems, the present inventors have examined a way for directly introducing an aromatic hydrocarbon compound in the liquid form to a reduction electrode side of electrolysis cell. In this case, as compared with a method of introducing a vaporized aromatic hydrocarbon compound, the above-described way allows an electrolytic hydrogenation reaction in a high current density to be performed. However, for such a way, when the current density exceeds any value, the electrolytic hydrogenation reaction and the hydrogen generation reaction are competed, and thereby, there is a problem in that Faraday efficiency that is a yield of electrolytic hydride per the quantity of flowing electricity decreases.